1. Field of the Invention
The present invention relates generally to the field of circulating fluidized bed (CFB) reactors or boilers such as those used in industrial or electric power generation facilities and, in particular, to a non-mechanical valve for controlling solids discharge from an in-bed heat exchanger (IBHX) to the CFB.
2. Description of the Related Art
U.S. Pat. No. 6,532,905 to Belin et al. describes a CFB boiler with controllable IBHX. The boiler comprises a CFB reaction chamber as well as a bubbling fluidized bed (BFB) heat exchanger located inside the reaction chamber. Heat transfer in the heat exchanger is controlled by means of controlling the rate of solids discharge from the lower part of the BFB into the reaction chamber. In one embodiment, the discharge control is accomplished using at least one non-mechanical valve that is controlled via the supply of fluidizing gas in the vicinity of the valve.
Another method for controlling the heat transfer is disclosed in U.S. Pat. No. 6,532,905. In this instance, heat transfer is controlled by using one or more conduits extending from a lower part of a BFB to an upper level at or above the lowest portion of the walls forming an IBHX enclosure. By fluidizing the solids particles in the conduit, their upward movement through the conduit is promoted, causing the solids particles to be discharged from the BFB into the surrounding CFB. By controlling the fluidizing gas flow rate, or the number of conduits in operation, the overall solids discharge from the BFB to the CFB is controlled, thus controlling heat transfer in the IBHX.
The higher the capacity of the CFB boiler and/or its exit steam parameters, the higher is the required heat duty of its IBHX. This is even more pronounced in an oxy-firing CFB boiler with elevated oxygen concentration, where the required heat duty of an IBHX for a given reaction chamber size increases drastically resulting in the increased height of the IBHX. Due to higher density of the BFB versus CFB, pressure differential across the non-mechanical valve may reach tens of inches of water column resulting in a high velocity of solids discharge through the valve and overall high flow rate of discharge. The latter may exceed a required rate of solids throughput and thus can adversely affect the controllability of the heat transfer. High solids velocity in the vicinity of the solids control valve may cause erosion of any adjacent tubes of the heating surface in the heat exchanger, as well as erosion of the bubble caps in the CFB reaction chamber in the wake of the jet from the valve.
Given the above, a need exists for a solids control valve that improves the operability and reliability of a CFB boiler where such a boiler contains a controllable IBHX.